Last ISMS, we reported on Stark effects of the ν₃ band of methane observed with a sub-Doppler resolution spectrometer. We determined the rotation-induced permanent dipole moment (PEDM) in the vibrational ground state and the vibration-, rotation-, and Coriolis-type-interaction-induced PEDMs in the v₃=1 state. r9cm Figure Figure illustrates Stark modulation spectrum of the Q(6)E with the external electric field of 31.0 kV/cm and the selection rule of ∆M=±1, where M is the magnetic quantum number. The ∆M=1 and −1 components of the Lamb dips labeled by A and B are resolved, and the central component C is identified with the cross-over resonance. The Lamb dips are assigned to the magnetic quantum numbers of the lower and upper states, (M",M′) according to the Clebsch-Gordan coefficients. We found that the relative intensity of the cross-over resonance to the associated Lamb dips depends on the P, Q, and R branches. We ascribe the dependence to the collisional relaxation processes.

Formaldehyde is the smallest stable organic molecule containing the carbonyl functional group, and is commonly considered to be a prototype for the study of high-resolution spectroscopy of polyatomic molecules. The a-axis Coriolis interaction between the near-degenerate ν₄ and ν₆ (out-of-plane and in-plane wagging modes, respectively) of the ground electronic state has received extensive attention and is thoroughly understood. In the first excited singlet <̃span class="roman">A ¹A₂ electronic state, the analogous Coriolis interaction does not occur, because the <̃span class="roman">A state suffers from a pseudo Jahn-Teller distortion, which causes a double-well potential energy structure in the q₄′ out-of-plane coordinate, and which dramatically reduces the effective ν₄′ frequency. The ν₄′ frequency is reduced by so great an extent in the <̃span class="roman">A state that it is the 3ν₄′ overtone which is near degenerate with ν₆′. In the current work, we report the precise ν₆′ fundamental frequency in the <̃span class="roman">A state, and we determine the strength of the a-axis Coriolis interaction between 3ν₄′ and ν₆′. We also provide a rotational analysis of the ν₄′+ν₆′ combination band, which interacts with 3ν₄′ via an additional c-axis Coriolis perturbation, and which allows us to provide a complete deperturbed fit to the 3ν₄′ rotational structure. Knowledge of the Coriolis interaction strengths among the lowest-lying levels in the <̃span class="roman">A state will aid the interpretation of the spectroscopy and dynamics of many higher-lying band structures, which are perturbed by analogous interactions.

The formaldehyde molecule is an important model system for understanding dynamical processes in small polyatomic molecules. However, prior to this work, there have been no reports of a resonance-enhanced multiphoton ionization (REMPI) detection scheme for formaldehyde suitable for rovibrationally state-selective detection in molecular beam scattering experiments. Previously reported tunable REMPI schemes are either non-rotationally resolved, involve multiple resonant steps, or involve many-photon ionization steps. In the current work, we present a new 1+1′ REMPI scheme for formaldehyde. The first photon is tunable and provides rotational resolution via the vibronically allowed <̃span class="roman">A ¹A₂←<̃span class="roman">X ¹A₁ transition. Molecules are then directly ionized from the <̃span class="roman">A state by one photon of 157 nm. The results indicate that the ionization cross section from the 4¹ vibrational level of the <̃span class="roman">A state is independent of the rotational level used as intermediate, to within experimental uncertainty. The 1+1′ REMPI intensities are therefore directly proportional to the <̃span class="roman">A←<̃span class="roman">X absorption intensities and can be used for quantitative measurement of <̃span class="roman">X-state population distributions.

Six studies have been recently devoted to a systematic analysis of the high-resolution near infrared absorption spectrum of acetylene recorded by Cavity Ring Down spectroscopy (CRDS) in Grenoble and by Fourier-transform spectroscopy (FTS) in Brussels and Hefei. On the basis of these works, in the present contribution, we construct an empirical database for acetylene in the 5850 - 9415 cm⁻¹ region excluding the 6341-7000 cm⁻¹ interval corresponding to the very strong ν₁+ ν₃ manifold. The database gathers and extends information included in our CRDS and FTS studies. In particular, the intensities of about 1700 lines measured by CRDS in the 7244-7920 cm⁻¹ are reported for the first time together with those of several bands of ¹²C¹³CH₂ present in natural isotopic abundance in the acetylene sample. The Herman-Wallis coefficients of most of the bands are derived from a fit of the measured intensity values. A recommended line list is provided with positions calculated using empirical spectroscopic parameters of the lower and upper energy vibrational levels and intensities calculated using the derived Herman-Wallis coefficients. This approach allows completing the experimental list by adding missing lines and improving poorly determined positions and intensities. As a result the constructed line list includes a total of 10973 lines belonging to 146 bands of ¹²C₂H₂ and 29 bands of ¹²C¹³CH₂. For comparison the HITRAN2012 database in the same region includes 869 lines of 14 bands, all belonging to ¹²C₂H₂. Our weakest lines have an intensity on the order of 10⁻²⁹ cm/molecule,about three orders of magnitude smaller than the HITRAN intensity cut off. Line profile parameters are added to the line list which is provided in HITRAN format. The comparison to the HITRAN2012 line list or to results obtained using the global effective operator approach is discussed in terms of completeness and accuracy.

One-color (212-220 nm) multi-photon photodissociation of acetylene, resonantly enhanced by the \̃textA(S₁)−\̃textX transition, gives rise to strong photofragment fluorescence signals in the visible and near UV regions. In this work, fluorescence signals from the photofragments, generated with three intermediate S₁ levels (trans 3⁴, trans 3⁵, and cis 3¹6¹), are studied, both in the flow cell and supersonic jet conditions. In the flow cell ( ∼ 3 torr), the dispersed fluorescence (DF) spectra of the photofragments are obtained. For all three S₁ levels, we observe C₂ Swan band (d³Π_g−a³Π_u) and C₂ Deslandres-d'Azambuja band (C¹Π_g−A¹Π_u) emissions, with the former approximately four times more intense than the latter. In the supersonic jet condition (collision-free), fluorescence time-traces at selected wavelength regions are analyzed. We confirm the presence of the two C₂ emission bands and their relative intensity observed in the DF spectra. In the supersonic jet condition, we also observe long lifetime visible fluorescence signal ( > 3 μs lifetime), which is likely due to emissions from C₂H fragment, based on previous vacuum UV photolysis studies of acetylene. The photodissociation mechanism is inferred, based on our analysis of the flow cell DF spectra and the fluorescence time-traces obtained in the supersonic jet condition. The C₂H fragment is likely generated from one-photon photodissociation of S₁ acetylene, and an additional photon dissociates the C₂H fragment into the C₂ C and d states.

Partially fluorinated carboxylic acids exhibit rich conformational landscapes. We report the first high-resolution spectroscopic study of 3,3,3-trifluoro-2-(trifluoromethyl)propanoic acid. Its rotational spectrum was measured using both broadband chirped-pulse and narrow-band cavity-based Fourier transform microwave spectrometers. Two dominant conformers were observed, and their structures confirmed with the aid of quantum chemical calculations. Both conformers take on the Z form of the carboxylic acid group. Similarities and differences between this and other fluorinated carboxylic acids are discussed.

We have recently shown that gauche-1,3-butadiene is unambiguously non-planar, with a C=C−C=C dihedral angle of about 34^°, and readily tunnels between two equivalent gauche structures.M.-A. Martin-Drumel et al., ISMS 2016, MI11n this talk, subsequent microwave studies of gauche-1,3-butadiene and its isotopologues as well as the empirical equilibrium structure will be summarized. The experiments have utilized the complementary techniques of cavity enhanced Fourier transform microwave (FTMW) spectroscopy with a supersonic expansion and chirped-pulse FTMW in a cryogenic buffer gas cell. The structural characterization is complicated by the effects of facile tunneling, and full dimensional ab initio rotational-VMP2 calculations have been performed to address this issue. We will show how the tunneling splitting frequency, which ranges between about 0.5 and 2.0\text cm⁻¹ (depending on the isotopologue), can be extracted from the experimental spectra by careful examination of tunneling-rotation perturbations.

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M.-A. Martin-Drumel et al., ISMS 2016, MI11I

As part of our project on the study of isotopologues of propane we have taken the spectra of the 2-D and 2,2-D₂ substituted species. There have been no studies of these species since the early IR studies. Friedman & Turkevich, J. Chem. Phys. 17, 1012 ff. (1949); McMurry, Thornton & Condon, J. Chem. Phys. 17, 918 ff. (1949).McMurry & Thornton, J. Chem. Phys. 19, 1014 ff.(1951). Gayles & King, Spectrochim. Acta 21, 543 ff.(1965).Kondo & Saeki, Spectrochim. Acta 29A, 735 ff. (1973). We recorded high resolution (∆ν = 0.0009 cm⁻¹) FTS data on the Canadian Light Source Far-IR beamline. The spectra of all bands of both species in the region examined (500 - 1250 cm⁻¹) show torsionally perturbed lines, all but one band appearing globally perturbed. Virtually all bands were not amenable to analysis at present except for the ν₁₅ (B₁) A-type band centered at 954.709 cm⁻¹. One can still see a few perturbed lines with torsional components but overall most lines were single and could be readily assigned using traditional methods. The spectrum is modelled well using PGOPHER.Western, J. Quant. Spectrosc. Rad. Transf. 186, 221 ff. (2017).o MW determined GS constants were available so we have analyzed about 3500 levels to determine both ground state and upper state rotational constants.

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Footnotes:

Friedman & Turkevich, J. Chem. Phys. 17, 1012 ff. (1949); McMurry, Thornton & Condon, J. Chem. Phys. 17, 918 ff. (1949).

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Gayles & King, Spectrochim. Acta 21, 543 ff.(1965).

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Western, J. Quant. Spectrosc. Rad. Transf. 186, 221 ff. (2017).N

Imidazole ring is a part of many biologically important molecules and drugs. Imidazole monomer, dimer and its complexes with water have earlier been studied using infrared spectroscopy in helium droplets^1,2 and molecular beams³. These studies were focussed on the N-H and O-H stretch regions, covering the spectral region of 3200-3800 cm⁻¹. We have extended the studies on imidazole clusters into the ring vibration region. The imidazole clusters were isolated in helium droplets and were probed using a combination of infrared spectroscopy and mass spectrometry. The spectra in the region of 1000-1100 cm⁻¹and 1300-1460 cm⁻¹were recorded using quantum cascade lasers. Some of the observed bands could be assigned to imidazole monomer and higher order imidazole clusters, using pickup curve analysis and ab initio calculations. Work is still in progress. The results will be discussed in detail in the talk.
References: 1) M.Y. Choi and R.E. Miller, J. Phys. Chem. A, 110, 9344 (2006). 2) M.Y. Choi and R.E. Miller, Chem. Phys. Lett., 477, 276 (2009). 3) J. Zischang, J. J. Lee and M. Suhm, J. Chem. Phys., 135, 061102 (2011). Note: This work was supported by the Cluster of Excellence RESOLV (Ruhr-Universitat EXC1069) funded by the Deutsche Forschungsgemeinschaft.